In many aspects elongated members of wind turbines are exposed to stress that causes strain. For example, the tower of a wind turbine as well as the wind turbine rotor blades may be exposed by strong wind which causes a strain on the tower and the rotor blades. The load on the rotor blades and/or the tower may be reduced by a variation of the blade's pitch. This can be realised by individual pitch controllers in general or by a cyclic pitch controller.
An effective pitch control, for example for reducing the loads acting on the rotor, needs information about the load acting on the blade root which can be extracted from a determination of the rotor blade deflection. Optical fibres in the blades are typical sensors for a reliable and long term measurement of the rotor loads. Such systems are rather expensive.
In U.S. Pat. No. 7,059,822 B2 a method for determining a rotor blade deflection is disclosed wherein a rotor blade is coupled with a hub. The rotor blade includes a beam with a first end coupled to a baffle inside the rotor blade, while the second end of the beam is located adjacent to the hub and is used for measuring the deflection of the beam by means of at least one sensor located in the hub. The beam is located near the centreline of the rotor blade. The determination of the deflection of the blade is based on the movement of the beam which correlates with the blade deflection.